Communication systems, such as conventional telephone communication systems, provide audio communication between two or more users during a communication session. Each user may communicate with each other using a communication device having a speaker and a microphone. During a communication session, the microphone of the communication device may experience echo (e.g., line echo, hybrid echo, acoustic echo). The communication device may send a signal corresponding to the echo from the speaker to a far-end user's communication device. If the signal transmitted by the communication device to the far-end user includes acoustical energy from the speaker, the far-end user may hear an “echo” of his or her own utterances. Conventional audio echo cancellation (AEC) techniques are designed to predict and remove the echo from an electrical signal generated by the microphone. In a line echo situation, the near end user's echo is being canceled.
FIG. 1 is a simplified block diagram of a conventional communication device 100. The communication device 100 includes a speaker 102 and a microphone 104 that are operably coupled with a double-talk detector 106 and an adaptive filter 108. In operation, the speaker 102 receives a speaker signal SPKR including audio from a far-end user and converts the speaker signal SPKR to sound waves for the near-end user to hear. The microphone 104 generates a microphone signal MIC that is based on the sound waves picked up by the microphone 104. The sound waves picked up by the microphone 104 may include the utterances from the near-end user as well as an echo of the speaker signal SPKR.
The adaptive filter 108 may include an echo predictor 110 that is configured to calculate a predicted echo ECHO′. The adaptive filter 108 may subtract the predicted echo ECHO′ from the microphone signal MIC to generate a filtered microphone signal FILT that has at least a portion of the echo removed. Ideally, the filtered microphone signal FILT has the full echo from the speaker signal SPKR removed such that the filtered microphone signal FILT includes only the voice of the near-end user, which may be transmitted to the far-end communication device without echo. The filtered microphone signal FILT may sometimes be referred to as an “error signal” during periods of speech or non-speech because it is the residual energy after cancelation.
The adaptive filter 108 may be configured to model the microphone echo signal ECHO to generate the predicted echo ECHO′, which may be subtracted from the microphone signal MIC. The adaptive filter 108 may monitor the speaker signal SPKR and the filtered microphone signal FILT to update the adaptive filter 108 to reduce the error from the predicted echo ECHO′. The adaptive filter 108 may continue to remove the predicted echo ECHO′ from the microphone signal MIC, and the filtered microphone signal FILT may be sent to the far-end user.
During a communication session, there may be a “double-talk” situation, in which the near-end user and the far-end user both speak at the same time. If the adaptive filter 108 updates during double-talk, the adaptive filter 108 may use the voice of the near-end user to model the microphone echo, which may result in the adaptive filter 108 may diverging predicting an inaccurate echo. Such divergence may, in some situations, increase the amount of echo heard by the far-end user or otherwise contribute to an unintelligible signal heard by the far-end user.
The double-talk detector 106 may be configured to detect double-talk and prevent the adaptive filter 108 from updating during double-talk. For example, the double-talk detector 106 may send a double-talk signal DT to the adaptive filter 108, indicating that double-talk has been detected. During double-talk the adaptive filter 108 may not update. Conventional double-talk detectors 106 may fail to detect double-talk according to some probability of miss. As a result, the adaptive filter 108 may, at times, update during double-talk, which may cause the adaptive filter 108 to diverge. To reduce the effects of this probability of miss, conventional double-talk detectors 106 may be configured to assert the double-talk signal DT for a predetermined amount of time (e.g., 40 ms) after detecting double-talk. This conventional approach may result in some improvements in reducing instances of updating the adaptive filter 108 during double-talk; however, the inventor has appreciated that further improvements may still be desirable.